This invention relates to a Pb alloy insoluble anode, and particularly to a Pb alloy which exhibits superior durability as an insoluble anode in electroplating using a sulfuric acid-type plating bath and in electrolytic refining using a sulfuric acid-type bath. The Pb alloy insoluble anode is advantageous especially when used as an anode in a high-speed, high current density process.
Therefore, this invention also relates to a continuous method for electroplating steel strip with zinc, and in particular to a continuous method for electroplating of zinc on steel strip in a high-speed line with a high current density, using the above-mentioned Pb alloy insoluble anode.
In general, when employing insoluble electrodes, plating metal is supplied in the form of ions. A plating metal salt is used as a raw material therefor and is usually dissolved in a plating solution and then supplied. For this reason, in order to increase the solubility of the salt, it is common to adjust the pH of the plating bath to a low level (for example, a pH of 1.0-2.0).
The low pH of the plating bath is a cause of corrosion of the discharge surface of insoluble electrodes. Therefore, even if there was no damage due to contact with the strip, the life span of the electrode was extremely short.
For example, even for a Ag-Pb alloy electrode which is said to have a relatively long life span, if zinc plating is carried out in a sulfuric acid bath with a pH of 2.0, the life span of the electrode is a maximum of 6 months.
Thus, in the past, alloys used for electrodes did not have adequate corrosion resistance, so the electrodes had to be frequently replaced.
Many methods for solving this problem have been proposed. For example, Japanese Laid-Open patent application No. 59-28598 discloses that if a small amount of In is added to a Pb(Ag-Pb) alloy for use as an insoluble anode in a sulfuric acid-type plating bath, the dissolution of the surface layer is remarkably decreased.
Namely, under actual plating conditions (bath composition: 250 g/l of FeSO.sub.4, 125 g/l of ZnSO.sub.4, 75 g/l of Na.sub.2 SO.sub.4 ; pH=2, bath temperature=60.degree. C., current density=30-45 A/dm.sup.2, material being plated=mild steel sheet, operating time=6 months), the average depth of corrosion was compared for a conventional material (2% Ag-Pb) and materials to which In was added. It was reported that the amount of corrosion was 32% that of the conventional material for a 5% In-Pb alloy and was 6.4% that of the conventional material for a 5% In-2% Ag-Pb alloy.
Similarly, in Japanese Laid-Open patent application No. 59-28599, an example is disclosed in which, instead of In, 0.5-10% of Sn is added to a Pb alloy containing 0.5-5% of Ag. In this case, under similar conditions, the average depth of corrosion decreased to 3% that of a comparative example. This is roughly half the amount of corrosion of a material to which In is added.
In addition, Japanese Laid-Open patent application No. 59-173297 discloses an example of a Pb alloy in which 0.1-3% of Sr is added to 0.5-5% of Ag, and Japanese Laid-Open patent application No. 58-199900 discloses an example of a Pb alloy in which 0.8-6% of Tl is added to 0.3-6% of Ag.
In order to maintain a high line speed so as to increase productivity, the trend in the design of recent electroplating cells is to employ higher current densities. For example, high current density plating at 150 A/dm.sup.2 or even 250 A/dm.sup.2 with a Ag-Pb alloy electrode has come to be actually performed.
Furthermore, as is apparent from the foregoing, in electrolytes used for electroplating and electrolytic refining, insoluble anodes are used, and electrolysis of Zn, Sn, Ni, Pb, Co, Fe, and Cu or alloys of these materials is performed on the surface of the material to be coated which serves as a cathode. Insoluble anodes made of Pb are commonly used. This is because Pb has corrosion resistance with respect to electroplating baths and electrolytic refining baths. Furthermore, when current is passed through Pb, lead oxide (PbO.sub.2) is formed on its surface, and this PbO.sub.2 functions as an insoluble anode. However, as Pb is not completely insoluble, when using a Pb insoluble anode, a small quantity of Pb dissolves. Furthermore, the PbO.sub.2 which is formed on the surface has poor adhesion to the Pb base, so the PbO.sub.2 may peel and does not always exhibit satisfactory durability, i.e., a satisfactory working life. At the same time, the small quantity of Pb.sup.2+ which dissolves in the electrolyte is electroplated together with Zn.sup.2+ and the like, and Pb is included in the electroplated film or in the electrolytically refined metal, decreasing the corrosion resistance of the plating or decreasing the purity of the refined metal.
The present inventors disclosed an insoluble anode having superior durability for use in electroplating on the surface of a metal in a sulfuric acid-type electroplating bath (see Japanese Patent No. 1300021, Japanese Published patent application Nos. 60-45719 and 60-45718, and Japanese Laid-Open patent application Nos. 59-173297 and 60-26635).
However, in these conventional insoluble anodes, under the present-day operating conditions employing a high current density such as 50 A/dm.sup.2 and even 100 A/dm.sup.2 and above, a small amount of Pb dissolves in the electroplating bath, particularly during high-speed plating. This is a main obstacle to performing high-quality electroplating. Moreover, at present there are increasing demands for reductions in production costs. Thus, there is a demand for anodes having a longer life span so as to decrease the length of time between anode replacement, thereby reducing the costs of the anodes themselves and reducing the down-time required for their replacement.
In a type of high current density plating method on a high-speed line, as in the above, the following characteristics are desired.
(i) Prevention of a decrease in the surface layer o insoluble electrodes. PA1 (ii) A decrease in the separation between electrodes. PA1 (iii) Inexpensive electrodes having a long life span. PA1 (iv) An increase in the quality of the plating. PA1 (v) Low power consumption.
These characteristics are all mutually related, but in particular, the higher the current density, the greater is the tendency for the electrode surface layer to decrease. Furthermore, the pH at the electrode interface further decreases (H.sub.2 O.fwdarw.2H.sup.+ +1/2(O.sub.2)+2e.sup.-), and the corrosive environment expands. Therefore, in a high-speed zinc electroplating line, a more effective measure to prevent the decrease of the electrode surface layer is needed.
Furthermore, the primary objective of performing high current density plating on a high-speed line is to increase productivity, i.e., output. However, if the current density is increased whereby the line speed is increased, the power consumption is proportional to the square of the current density, so the unit electric power consumption (the electric consumption per unit of production) increases in proportion to the increase in current density or line speed. Therefore, in order to perform high current density operation, a means of further reducing power consumption is necessary.